Most were only 1-2 mpg out, but some were found to be significantly awry--the Soul was 6 mpg overstated.

According to The Detroit News, both automakers will reimburse the 900,000 U.S. customers who bought the models, from 2011-2013 model years.

The reimbursement will be based on the period over which the owners have had the vehicles, and reflect the gas they would have saved had the cars hit the stated mileage--plus 15 percent.

Hyundai says a typical owner in Florida doing 15,000 miles per year could get an $88 refund this year, plus refunds on subsequent years, for as long as they own the vehicle. Figures will vary depending on the discrepancy and the owner's driving habits.

Owners will have their odometers read at dealerships and be reimbursed accordingly--and they'll be able to return for further readings as often as they like. Owners will not have to sign a release or waiver to participate.

Hyundai and Kia are also both reimbursing dealers for the readings, to the tune of $10 for each reading.

Owners have until the end of 2013 to register for compensation. The official mileage figures of each company have since been adjusted to the true figure

Overall, the bill for both automakers could be over $100 million--and the EPA is yet to decide whether further civic penalties are required.